Robin, who told you that?
A piano tuner has no problem tuning the higher notes of a piano.
there is no reason why GD cannot be tuned as accurately as a CG.
The most out of tune note in equal temperament is the third note of the chord,
which is why piano tuners, tune using circles of fifths example all the c notes are tuned then all the g notes then all the d notes etc.
in equal temperament the most in tune notes are fifths